The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Advanced Thermoelectric Design: From Materials and Structures to Devices

Textiles have been concomitant of human civilization for thousands of years. With the advances in chemistry and materials, integrating textiles with energy harvesters will provide a sustainable, environmentally friendly, pervasive, and wearable energy solution for distributed on-body electronics in the era of Internet of Things. This article comprehensively and thoughtfully reviews research activities regarding the utilization of smart textiles for harvesting energy from renewable energy sources on the human body and its surroundings. Specifically, we start with a brief introduction to contextualize the significance of smart textiles in light of the emerging energy crisis, environmental pollution, and public health. Next, we systematically review smart textiles according to their abilities to harvest biomechanical energy, body heat energy, biochemical energy, solar energy as well as hybrid forms of energy. Finally, we provide a critical analysis of smart textiles and insights into remaining challenges and future directions. With worldwide efforts, innovations in chemistry and materials elaborated in this review will push forward the frontiers of smart textiles, which will soon revolutionize our lives in the era of Internet of Things.

Smart Textiles for Electricity Generation.

Flexible thermoelectric materials and devices

Polymer based thermoelectric nanocomposite materials and devices: Fabrication and characteristics

In present-day high-performance electronic components, the generated heat loads result in unacceptably high junction temperatures and reduced component lifetimes. Thermoelectric modules can, in principle, enhance heat removal and reduce the temperatures of such electronic devices. However, state-of-the-art bulk thermoelectric modules have a maximum cooling flux qmax of only about 10 W cm−2, while state-of-the art commercial thin-film modules have a qmax <100 W cm−2. Such flux values are insufficient for thermal management of modern high-power devices. Here we show that cooling fluxes of 258 W cm−2 can be achieved in thin-film Bi2Te3-based superlattice thermoelectric modules. These devices utilize a p-type Sb2Te3/Bi2Te3 superlattice and n-type δ-doped Bi2Te3−xSex, both of which are grown heteroepitaxially using metalorganic chemical vapour deposition. We anticipate that the demonstration of these high-cooling-flux modules will have far-reaching impacts in diverse applications, such as advanced computer processors, radio-frequency power devices, quantum cascade lasers and DNA micro-arrays. Current thermoelectric modules provide cooling fluxes that are insufficient for high-heat flux applications. Here, the authors demonstrate thin-film-based thermoelectric modules capable of providing cooling fluxes more than double that of the current state-of-the-art.

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Superlattice-based thin-film thermoelectric modules with high cooling fluxes

Thin-film Bi2Te3- and Sb2Te3-based superlattice (SL) thermoelectric (TE) devices are an enabling technology for high-power and low-temperature applications, which include low-noise amplifier cooling, electronics hot-spot cooling, radio frequency (RF) amplifier thermal management, and direct sensor cooling. Bulk TE devices, which can pump heat loads on the order of 10 W/cm2, are not suitable in these applications due to their large size and low heat pumping capacity. Recently, we have demonstrated an external maximum temperature difference, ΔTmax, as high as 58 K in an SL thin-film p–n couple. This state-of-the-art couple exhibited a cold-side minimum temperature, Tcmin, of −30.9°C. We regularly attain ΔTmax values in excess of 53 K, in spite of the many significant electrical and thermal parasitics that are unique to thin-film devices. These measurements do not use any complex thermal management at the heat sink to remove the heat flux from the TE device’s hot side. We describe here multistage SL cooling technologies currently being developed at RTI that can provide useful microcooling cold-side temperatures of 200 K. This effort includes a three-stage module employing independently powered stages which produced a ΔTmax of 101.6 K with a Tcmin of −75°C, as well as a novel two-wire three-stage SL cascade which demonstrated a Tcmin of −46°C and a ΔTmax of nearly 74 K. These RTI modules are only 2.5 mm thick, significantly thinner than a similar commercial three-stage module (5.3 mm thick) that produces a ΔTmax of 96 K. In addition, TE coolers fabricated from these thin-film SL materials perform significantly better than the extrapolated performance of similar thickness bulk alloy materials.

Three-Stage Thin-Film Superlattice Thermoelectric Multistage Microcoolers with a ΔTmax of 102 K

A thermoelectric structure including a thermoelectric material having a thickness less than 50 nm and a semi-insulating material in electrical contact with the thermoelectric material. The thermoelectric material and the semi-insulating materials have an equilibrium Fermi level, across a junction between the thermoelectric material and the semi-insulating material, which exists in a conduction band or a valence band of the thermoelectric material. The thermoelectric structure is for thermoelectric cooling and thermoelectric power generation.

Nanoscale, ultra-thin films for excellent thermoelectric figure of merit

Metallorganic chemical vapor deposition of InAs/GaSb superlattices on GaAs substrates using a two-step InAs buffer layer

We report on our investigation into the use of III-V superlattice structures for thermoelectric (TE) applications. Preliminary review of III-V materials trends indicate that the GaSb/InAs superlattice system should offer one of the best potentials for high thermoelectric performance in the 500K-800K range. MOCVD growth of GaSb/InAs superlattice structures was carried out, and relevant structural, thermal, and electrical characterization has been performed. TEM and XRD results demonstrate a well-ordered superlattice structure. Thermal conductivity measurements reveal a reduction in the room-temperature thermal conductivity of GaSb/InAs superlattices (4.4-10.0 W/m-K), relative to either binary GaSb (32 W/m-K) or InAs (27 W/m-K). Additionally, we have worked to optimize the thermoelectric power factor (α2σ), studying both Se- and Te-doping of the superlattice structures, in an effort to demonstrate optimal thermoelectric performance. Our results demonstrate a maximum ZT of 0.36 at 400K for optimally doped n-type GaSb/InAs superlattice structures.

Gary E. Bulman

Papers

Superlattice-based thin-film thermoelectric modules with high cooling fluxes

Three-Stage Thin-Film Superlattice Thermoelectric Multistage Microcoolers with a ΔTmax of 102 K

Nanoscale, ultra-thin films for excellent thermoelectric figure of merit

Metallorganic chemical vapor deposition of InAs/GaSb superlattices on GaAs substrates using a two-step InAs buffer layer

Investigation into the thermoelectric properties of GaSb/InAs superlattice structures